(1) Field of the Invention
This invention relates to microwave ovens for cooking food, and particularly to such ovens having a simmer cycle for cooking food slowly in a closed container at a simmer temperature for many hours of time.
(2) Description of the Prior Art
Microwave ovens have become widely accepted in many countries for the cooking of many foods at a fast cooking rate. The microwave frequency energy is radiated within the oven cooking cavity from an energy source such as a magnetron. The waves are radiated and reflected within the oven cavity in free space and are distributed by such means as mode stirrers, antennas, and the like. The microwave energy sets up a high-frequency oscillatory movement of the molecules in the food to cause internal heating by molecular friction.
Electric thermometer probes which monitor the internal temperature of the food while it is being cooked have been developed in recent years for use in microwave ovens. One example of such automatic temperature controls adapted for use in microwave ovens is taught in U.S. Pat. No. 3,975,720 of David Y. Chen and Louis H. Fitzmayer, which is assigned to the same assignee as is the present invention. Such probes comprise a long needle-like probe adapted to be positioned in the food, and with a temperature-sensing element, such as a thermistor, positioned internally of the probe housing near the tip thereof. A flexible shielded cable connects the thermistor to control circuitry which is responsive to thermally-induced changes in its resistance. The cable shield is connected at one end to the probe housing and at the other end to a wall of the oven cooking cavity for grounding purposes. The design of this probe and its cable has been carefully selected so that they may successfully be operated in a microwave oven with minimal direct heating of the probe and cable by the microwave energy. Prior to the Chen/Fitzmayer invention, microwave cooking had been accomplished using timed cycles. It was necessary to weigh the food and measure its thickness or depth and set the correct power level before attempting to estimate the correct amount of time for cooking the food. Only successful experiences would make a person confident enough to avoid overcooking the food. The introduction of the automatic temperature control of the Chen/Fitzmayer invention allows the food to be cooked to a pre-set temperature so that the cooking operation is automatic and the guesswork has been eliminated. When the pre-set internal food temperature is reached, the oven automatically turns itself off and a buzzer sounds to signal that the food is ready for serving. Electric meat thermometers have been used before in standard electric and gas cooking ovens, but an automatic temperature control is much more important in a microwave oven because once the microwave power is de-energized, the cooking operation ceases immediately. In standard electric and gas ovens, the cooking action continues even after the electric heaters are de-energized and the gas burners are extinguished unless the food is removed from the oven. This is true because of the stored heat energy in the electric heating elements or the gas burners and the heat in the oven walls and insulation, and the cooking container. Microwave cooking is cool cooking because the heating effect takes place directly in the food throughout simultaneously, and not from the heat of the container for the food or from the heat of the walls of the oven liner and oven door.
While microwave oven cooking is the fastest growing segment of the range industry, some people are reluctant to purchase a microwave oven because they feel no urgent need to cook faster and, furthermore, they do not perceive fast cooking to be good cooking of the type they desire to routinely serve their family. On the other hand, the majority of people attribute slow cooking and simmering with high quality end results of nutrition, tenderness, flavor, economy and convenience, ever with cheaper cuts of meat. Market surveys indicate that there is very little routine main course meal preparation with the microwave oven. Owners tend to use it often but in a very limited way.
Another fast growing branch of the food preparation equipment industry is the modern electric slow-cooking pots where it is possible to enjoy delicious simmered-in flavors of many popular dishes without the time-consuming necessity of constant attention. It is possible to set the slow-cooking pots in operation and leave for work or spend the day away from the home, or for cooking during the night for use the next day. The slow-cooking pots mingle the flavors and the spices and retain many of the vitamins that high temperatures destroy. It is possible to return home to a piping hot dinner that is ready to serve, and it doesn't matter if you return at any exact hour. The food won't burn or taste overcooked if it is heated several hours longer than planned. Dinner is ready whenever it is desired. Nourishing meals can be prepared with inexpensive meats because slow cooking tenderizes in a special way that broiling or frying cannot match. The meats are juicy and never cooked dry because slow-cooking pots seal in the moisture. Electric slow-cooking pots are popular because on a low setting it uses less energy than a 100-watt bulb, and it is possible to cook all day for only a few pennies. Accordingly, slow cooking makes good eating.
The principal object of the present invention is to provide a controlled temperature microwave oven with a slow simmer cycle in order to expand the utility of microwave oven cooking to be able to handle most kinds of food over a wide range of cooking methods while retaining the convenience of automatic temperature and timed controls, and variable power.
A further object of the present invention is to attain increased cooking versatility by combining the advantages of both microwave oven cooking and electric slow-cooking pots.
A further object of the present invention is to provide a microwave oven of the class described with a closed cooking container that is joined to the automatic temperature control system of the oven so that the food temperature is held at just below a boiling temperature for an unlimited period of time without overcooking.
A further object of the present invention is to provide a microwave oven with a simmer cycle using a closed cooking container that is transparent to microwave energy so that the food is heated directly by absorption of the microwave energy.
A further object of the present invention is to provide a microwave oven of the class described using an automatic temperature control system with a temperature-sensing probe associated with the bottom wall of the cooking container but not directly within the cooking vessel.
A further object of the present invention is to provide a slow-cooking microwave oven that is capable of defrosting frozen food and then simmer cooking the food in a single automatic operation.
A further object of the present invention is to provide a microwave oven of the class described where the temperature-sensing probe is inserted into a tunnel in the bottom wall of the cooking container for good thermal conduction of heat from the food load being cooked.